The invention described herein relates to nuclear reactors and more particularly to apparatus for loading fuel pellets into fuel rods prior to installation of the rods in a reactor fuel assembly.
Each fuel rod used in large nuclear reactors of the type designed for generation of electric power contains approximately 240 fuel pellets. These pellets conventionally measure 0.650 inch in length and 0.3674 inch diameter for insertion in a fuel rod having an internal diameter of 0.3719 inch. Such dimensions are typical and it will be understood that a different number and size of pellets may be used in reactors designed for other and different purposes.
Conventional pellet loading apparatus consists of a table supporting a tray having approximately 25 grooves, each about 2 feet long and of a size which accommodates fuel pellets of the above dimensions. Fuel rods on the same or a second, adjacent table are aligned with the grooves and the pellets are then pushed into the fuel rod opening either by hand or automatically by a machine designed for this purpose. As pellets travel through the rod, pellet jamming takes place with sufficient frequency as to impede and occasionally stop the fuel pellet loading operation. Exploration of the causes of jamming has led to the conclusion that pellets which do not have end faces perpendicular to the pellets centerline, i.e., non-square pellet ends, do not exert a uniform force on the end of the next pellet in the fuel rod stack. The angularity between adjacent pellet faces therefore causes the pellets to move off a line parallel with the fuel rod walls with the result that they buckle against the rod inner surface and produce a braking effect. As continued pressure is applied against the pellet stack in the fuel rod, high loading forces are generated which cause the pellet to turn and jam in the rod when acted on by another pellet. This action firmly locks the pellets in a jammed position thus precluding further pellet movement in the fuel rod. To alleviate this situation, it is necessary to vibrate the rod by any one of many different methods to break the jam loose. If this procedure is not successful, the fuel rod must be split open to effect pellet removal since the value of pellets is greater than the value of the fuel rod.